Body is divided into head (capitulum [contains mouthparts]), and body (idiosoma).

Mouthparts consist of structures that function in tactile sensing (palps), lacerating host skin (chelicerae), and anchoring the tick to the host (hypostome).

Body is covered by a complex waxy somewhat inflexible exoskeleton (tegument) that protects the tick from water loss and predators.

A scutum (dorsal shield) covers the entire surface of adult male tick, but only a portion of the dorsal surface of female ticks.

The scutum is sometimes colored with iridescent white or yellow patches (ornate tick) or may be devoid of such ornamentation (inornate ticks).

Partial coverage of the dorsal surface of female ticks allow for enormous expansion during feeding and engorgement.

Ticks have an excretory system (malpighian tubules), "liver" (fat body), an open circulatory system with a blood-like hemolymph, a respiratory system (net-like tracheae with openings called spiracles), a rather complex nervous system and a number of sensory structures that detect chemical, thermal, light and mechanical stimuli. A multifunctional organ located on the legs (Haller's organ) receives stimuli used during questing (crawling on vegetation to gain entry onto a host) and host detection. Some ticks have eyes.

Ixodid tick developmental cycles

• Ticks develop through four distinct life cycle stages.
o Egg, larva, nymph, adult
o Larvae, nymphs, and adults are similar in appearance, but differ in size and numbers of legs (larvae possess 6 legs; nymph and adults possess 8 legs).
o Stages increase in size from larva to adult.
o Larvae and nymphs are without features of sexual dimorphism. • Ticks that infest companion animals are 3-host ticks. • Most species utilize a different species of host at each stage; R. sanguineus utilizes the dog during each developmental phase. • Ixodes scapularis (Black-legged Tick; Deer Tick) may parasitize in excess of 100 host species representing three vertebrate classes (mammals, birds, reptiles).
• Certain ticks (i.e. R. sanguineus) can complete their life cycles in weeks, while others require two years or more. • Eggs are laid by replete females in sheltered environments off the host. • Larvae and each subsequent stage seek a host, feeds to repletion, drop from the host and either molts to next stage or deposit eggs (replete females).

Pathogens gain entry into a new host when the next stage (after molting) feeds on susceptible hosts (transstadial transmission) or by larvae that hatch from eggs through which the pathogen was passed (transovarian transmission).

Whether a tick will serve as a successful vector depends on several factors

o Size of bloodmeal ingested, virulence of pathogen, susceptibility of host.
o Capability of pathogen to infect and replicate in the tick.
o Successful dissemination of the pathogen throughout the tick's body.
o Transstadial or transovarial habits of the pathogen.
o Interactions between multiple microbes infecting the same tick.